1. Field of the Invention
The present invention relates to a screw for extruder provided with a bearing segment supporting a screw main body in order not to move the screw main body during kneading, the bearing segment used in the above screw for extruder, and a twin screw extruder provided with the above screw for extruder.
2. Description of the Related Art
In the case where a composite resin material is extruded, kneading is performed by adding a base material of a thermal plastic resin with an additive agent such as filler or a different synthetic resin. In such kneading, a continuous extruder is often used.
A known continuous extruder is provided with a barrel having a long hollow portion (chamber) along the axial direction and a screw body passing through the hollow portion in the axial direction, and is a single screw extruder or a twin screw extruder in accordance with the number of the screw body.
The continuous extruder is to supply a material to be kneaded into the barrel and rotate the screw body by a motor or the like so as to knead the material to be kneaded. In the screw body, a plurality of segments are coupled to each other in the axial direction, and specific flights are provided in the segments respectively. By the flights, the material to be kneaded is moved from the upstream side to the downstream side and a shear stress which is necessary for the kneading is given.
There are various configurations of the continuous extruder in accordance with a component of the material to be kneaded. However in general, at least one material supply port is provided in the barrel and various fillers, synthetic resins or the like are properly supplied from the material supply port. Therefore, in the screw body is formed a kneading portion provided with a plurality of kneading segments including a kneading disk segment having kneading flights with a large kneading effect and a rotor segment on the slightly downstream side of the material supply port. When the plurality of kneading portions are provided, there is sometimes a case where a plurality of material supply ports are provided in the axial direction in accordance with the kneading portions. In the screw body, is also provided a screw segment having a feeding flight which is suitable for feeding the material to be kneaded towards the downstream side. Then the kneaded material to be kneaded is moved to the downstream side, pressurized and extruded in an extruding portion installed in the vicinity of an end of the downstream.
The kneading is mainly progressed by receiving a shear effect when the material to be kneaded passes through a tip clearance formed between an inner wall of the barrel and a tip part of the kneading flight. Therefore, in order to improve a kneading effect (dispersibility), it is preferable that a gap (tip clearance) provided between the inner wall of the barrel and the tip part of the flights is properly narrowed. In order to improve a kneading effect (distributivity), it is preferable that the gap (tip clearance) is properly widened.
For example, US Patent Laid-Open No. 2006-3042 discloses a technique of a twin screw extruder in which kneading flights with two tip portions (hereinafter sometimes referred to as “streaks”) twisting in a spiral shape in the axial direction are adapted in a rotor segment. The flights have a high tip part having a large rotational outer diameter and a low tip part having a small rotational outer diameter. In the high tip part, a tip clearance to an inner wall of a barrel is small, and in the low tip part, the tip clearance is large.
The twin screw extruder is capable of achieving a preferable kneading effect.
However, in the twin screw extruder in general, a large kinetic load is also imposed on a kneading screw as an adverse effect of a high kneading effect. Particularly, a kinetic load imposed on both a kneading segment and a rotor segment of the screw body is very large.
As a result, an axial center of the screw body is bent and a rotational axis of the screw body is easily moved. In the screw body with the moving rotational axis, front ends of the flights are easily brought in contact with the inner wall of the barrel. It can be predicted that the above tendency is remarkable in a kneading portion provided with a kneading segment and a rotor segment having a small tip clearance.